EFFECTS OF THE DIELECTRIC CONSTANT 



813 



Table 15-4 

 Effects of Methanol ox the Kln'etic Constants of Papain " 



" From Stockell and Smith (195"; 

 what too high for the mixtures. 



The dielectric constants may possibly be some- 



the substrate. However, more complete data and a re-evaluation of the 

 meaning of Ky„ led Stockell and Smith more recently to explain the effects 

 in terms of the dielectric constant. In the first place, K„i was found to be 

 k^jki and not the substrate constant. "When log kolK,,^ was plotted against 

 1/Z), a straight line was obtained; this, of course, means that log k^ plotted 

 against 1 ID is linear and that the rate of formation of the ES complex does 

 behave as expected with regard to the variation of the dielectric constant. 

 In the second place, the use of isopropanol gave the same dependence of 

 the constants on the dielectric constant and yet isopropanol would not be 

 expected to hydrogen bond to the enzyme as readily as methanol. Since 

 ki decreases with a lowering of the dielectric constant, the formation of the 

 ES complex must involve interactions Ijetween ionic groups of like charge 

 or between dipoles oriented in the same direction. How this correlates with 

 the suggested reaction scheme, in which a positively charged substrate 

 group interacts with a negatively charged carboxylate group on the enzyme, 

 is unknown. 



{d) Pancreatic carboxy peptidase. The effects of several organic solvents 

 on the hydrolysis of carbobenzoxyglycyl-L-tryptophan by carbox>T)epti- 

 dase were studied by Lumry and Smith (1955) but the results were only 



